Is it possible that your mood can affect the potency of a vaccine? British researchers tested older adults ages 65-85. They recorded mood, stress level, negative thoughts, sleep patterns, and diet before administering a vaccine. Afterwards they followed with blood tests from 4 to 16 weeks. Only those who had exhibited a positive mood before the vaccine was administered, had higher levels of the antibodies to resist HINI, a potentially dangerous flu strain. It seems your mood on the day of being vaccinated has the most to do with how effective the outcome will be.